Is he cheating?

So what’s a good way to determine if your man is cheating on you? He used to like it when I grabbed his butt in the heat of the moment. Well last night he said you don’t have to do that anymore. On top of that we have been TTC but when I tell him I’m ovulating those are the days we DON’T have sex. The arguments have slightly increased and he’s a bit more withdrawn. Any thoughts or advice?